The legality of exhaust welding?

As long as no exhaust gas can escape - welding is fine!

I have taken my car for an MOT with a coke can wrapped around the exhaust before and it has passed - no exhaust gases escaping and no excessive noise and it will be fine!

From your question I assume your in the UK?

I am an MOT tester with over 20yrs of testing under my belt and when presented with a vehicle for retest that has failed originally for an exhaust fault, I apply the minimum test standards and assess that exhaust on condition, structural and security then sound, if the system has been welded has the repair created a problem and if so is it a testable item?, if not pass and advise. You can safely say it os ok to weld your exhaust.

It will fail the mot if there is a significant leak from the exhaust, If the leak is tiny or non existent then it is ok, even if you have patched a hole with a baked bean can, it will pass if there is no leak.

Welding is fine, the systems are all welded construction anyway

box or plate weld at least two inches around the hole , or on a curved section a shaped plate , provided the rest of the surrounding material is sound , most box too tube are welded from new to allow fitting in the correct position, all of the above is acceptable under MOT rules

Perfectly OK to weld providing it's done well and the exhaust is correctly supported. If done properly the MOT will not fail on construction grounds or emission groulds

There is no criteria in the Mot manual of how a hole should be repaired in an exhaust as long as it's durable enough and doesn't weaken the exhaust structure.

There is no problem welding up your exhaust, so long as it is a gas tight join you are doing nothing illegal

it is legal to weld exhaust holes. You can do as you wish to the vehicle if you drive it on private property only.

don't weld up the big hole in the end

As long as no exhaust gas can escape